Photopolymerizations used in UV curing processes are increasingly important reactions that are currently widely employed in many large-scale commercial applications such as coatings, adhesives, printing inks, and photoresists. At the present time, the applications of photopolymerization are experiencing growth in excess of 10% annually. Typically, such polymerizations are conducted under irradiation with UV light in the absence of a solvent and typically proceed within the time frame of a fraction of a second to several minutes. Multifunctional monomers are most commonly employed and crosslinked polymer networks are obtained. As the technical applications of photopolymerizations have multiplied, there has been a corresponding interest on the part of both academic and industrial workers in the development of various methods by which these very rapid polymerization reactions can be monitored. Convenient, precise and reproducible analytical methods for following the course of photopolymerizations are crucial not only to the future research and development of this technology, but are also the key to insuring the photoresponse of currently available commercial products.
A considerable number of analytical techniques have been developed for monitoring the course of rapid photopolymerization reactions. Only a few of the most commonly used techniques are presented here. Probably the oldest analytical method that has been employed for the study of photopolymerizations is calorimetry carried out in specially modified bomb calorimeters. A more recent development is the use of differential scanning photocalorimetry (DSP). This technique involves the adaptation of a conventional differential scanning calorimeter by the addition of a light source and a quartz window to allow the irradiation of samples placed in the calorimeter. The evolution of the exothermic heat of the polymerization of the monomer is used to monitor the course of the reaction. DSC instruments equipped with irradiation modules are now available from several commercial sources.
From the inception of the discovery and development of photoinduced polymerizations, infrared spectroscopy (IR) was used as an important method for characterizing the polymers produced. However, the most recent seminal innovation in this area was the development of real-time infrared spectroscopy (RTIR) by Decker et al. to enable the continuous, rapid, and precise monitoring of the kinetics very rapid photopolymerizations. Initially, this technique involved the observation of a single absorption band characteristic of the functional group undergoing polymerization as a function of time. More recently, highly sensitive, Fourier transform spectrometers have become available that allow the acquisition of tens of complete spectra per second. Such spectrometers provide the ability to monitor several bands of complex mixtures of monomers undergoing polymerization at the same time. At this time, FT-RTIR appears to be the method of choice for the rapid, precise monitoring of photopolymerization reactions. A related technique, in-situ Raman spectroscopy, has also been applied to photopolymerizations. Fluorescence spectroscopy has also been used as a technique for following the kinetics of photopolymerizations.
Yet, despite the plethora of methods that have been used and developed to monitor the course of photopolymerization reactions, there is still a need for new methodology that provides additional information concerning these reactions. Such methods should also be rapid, reproducible and easy to implement. In addition, the apparatus used should also be relatively inexpensive. A Perkin-Elmer Differential Scanning Calorimeter equipped with an irradiation module costs approximately $150,000 while a FTIR equipped with a lamp source is priced in the range $20,000–30,000. No commercially available RTIR equipment is available. For this reason, a FTIR spectrometer must be purchased and then custom modified to provide FT-RTIR capability. Many users of UV cure technology are small companies that cannot easily afford these kinds of expenditures for such equipment. Therefore, there is a need for new techniques and instruments for monitoring UV curable formulations that not only meets the objectives set forth above, but also are comparatively inexpensive.
All addition photopolymerizations are exothermic events and, as mentioned previously, the technique of differential scanning photocalorimetry relies on this principle. However, DSP suffers from serious drawbacks that limit its usefulness apart from the inherent high cost of this instrumentation. Results are highly dependent on the sample size and configuration. For this reason, the data obtained is often poorly reproducible. Since it is an indirect method, DSP gives very little information about the actual chemistry that is taking place. On the other hand, RTIR provides an excellent monitoring of the chemistry that is taking place during the chemical reaction, but fails to provide information about the simultaneous changes in the environment and physical state of the sample. For example, as a photopolymerization proceeds, the temperature of the sample must rise during the reaction. Little information is available on the magnitude of the temperature increase or its effect on the kinetics and extent of the polymerization.
Although RTIR and DSP work well when they are employed for the polymerization of simple one- or two-monomer systems, they are less easily applied to systems in which more complex multicomponent mixtures of monomers of different reactivity are involved. Further, they are comparatively slow and labor-intensive, and employ expensive instrumentation. As a consequence, they are rarely employed in an industrial setting for the optimization of photocurable formulations. Accordingly, there is also a need for highly sensitive, versatile, rapid, reproducible and easy to use analytical instruments for monitoring photopolymerization.
Optical pyrometers have been employed for a wide variety of remote temperature sensing purposes. For example, they are used in the metals, glass and ceramics industries to determine the temperature of these molten and solid materials during the various stages of their processing. However, instruments based on an optical pyrometer have not heretofore been used in monitoring rapid photopolymerizations.
Optical pyrometers are small and inexpensive instruments that can be easily mounted in various modes and configurations to record temperature by measuring the infrared emission of a sample. Typically, they provide a wide temperature measurement range capability with an accuracy of ±1° C. Further, the temperature measurements are rapid and can be made on either a continuous or discontinuous mode. U.S. Pat. No. 5,707,780 discloses use of an optical pyrometer for determining relative degree of polymerization over time in developing materials for use in a solid imaging process. Samples were scanned by a laser in a pattern of parallel lines to induce polymerization. Surface temperature was measured by an optical pyrometer, recorded on a strip chart and the area under the curve was used to indicate relative degree of polymerization for various samples. U.S. Pat. No. 6,268,403, to the present inventor, describes determining the rate of polymerization using FTIR and states that temperature of the sample was recorded using an optical pyrometer. Neither reference suggests building a stand-alone instrument that is robust, inexpensive and easy to use, and could be used in an industrial setting.